A Coriolis flowmeter is a mass flowmeter based on a point that a Coriolis force acting on a flow tube (hereinafter, flow tube to be vibrated is referred to as flow tube) is proportional to a mass flow rate in a case where the flow tube through which a fluid to be measured flows is supported at both ends and vibration is applied about a support point in a direction perpendicular to a flow direction of the flow tube. The Coriolis flowmeter is well known and a shape of a flow tube in the Coriolis flowmeter is broadly divided into a straight-tube type and a curved-tube-type.
The Coriolis flowmeter is a mass flowmeter for detecting a phase difference signal proportional to a mass flow rate in symmetrical positions between both end support portions and central portion of a measurement tube in a case where the measurement tube through which a fluid to be measured flows is supported at both ends and the central portion of the supported measurement tube is alternately driven in a direction perpendicular to a support line. The phase difference signal is the quantity proportional to the mass flow rate. When a driving frequency is maintained constant, the phase difference signal may be detected as a time difference signal in the observation positions of the measurement tube.
When the alternate driving frequency of the measurement tube is made equal to the natural frequency of the measurement tube, a constant driving frequency corresponding to a density of the fluid to be measured is obtained, and hence the measurement tube may be driven with small driving energy. Therefore, recently, the measurement tube is generally driven at the natural frequency and the phase difference signal is detected as the time difference signal.
The straight-tube type Coriolis flowmeter has a structure in which, in a case where vibration is applied in a direction perpendicular to a straight tube axis of a central portion of a straight tube supported at both ends, a displacement difference of the straight tube which is caused by a Coriolis force, that is, a phase difference signal is obtained between the support portion and central portion of the straight tube, and a mass flow rate is detected based on the phase difference signal. The straight-tube type Coriolis flowmeter as described above has a simple, compact, and tough structure. However, the Coriolis flowmeter also has a problem that high detection sensitivity cannot be obtained.
In contrast to this, the curved-tube-type Coriolis flowmeter is superior to the straight-tube type Coriolis flowmeter in the point that a shape for effectively taking out the Coriolis force may be selected. The mass flow rate may be actually detected with high sensitivity.
A combination of a coil and a magnet are generally used as driving means for driving the flow tube. The coil and the magnet are preferably attached to positions which are not offset in the vibration direction of the flow tube because a positional relationship deviation between the coil and the magnet is minimized. Therefore, in a case of a curved-tube-type Coriolis flowmeter including two parallel flow tubes, the two parallel flow tubes are attached so as to sandwich the coil and the magnet. Therefore, a design is made so that the two opposed flow tubes are separated from each other at an interval to sandwich at least the coil and the magnet.
Of Coriolis flowmeters including two flow tubes located in parallel planes, a Coriolis flowmeter having a large diameter or a Coriolis flowmeter having high flow tube rigidity is required to increase power of driving means, and hence it is necessary to sandwich large driving means between the two flow tubes. Therefore, a design is made so that an interval between the flow tubes is necessarily widened even in a fixed end portion which is a base portion of the flow tubes.
As illustrated in FIG. 10, a Coriolis flowmeter 1 which is generally known and includes U-shaped measurement tubes includes a detector 4 for two U-shaped measurement tubes 2 and 3, and a converter 5.
The detector 4 for the measurement tubes 2 and 3 includes a vibrator 6 for resonance-vibrating the measurement tubes 2 and 3, a left velocity sensor 7 for detecting a vibration velocity generated on a left side of the measurement tubes 2 and 3 vibrated by the vibrator 6, a right velocity sensor 8 for detecting a vibration velocity generated on a right side of the measurement tubes 2 and 3 vibrated by the vibrator 6, and a temperature sensor 9 for detecting a temperature of a fluid to be measured, which flows through the measurement tubes 2 and 3 at the detection of the vibration velocity. The vibrator 6, the left velocity sensor 7, the right velocity sensor 8, and the temperature sensor 9 are connected to the converter 5.
The fluid to be measured, which flows through the measurement tubes 2 and 3 of the Coriolis flowmeter 1, flows from the right side of the measurement tubes 2 and 3 (side on which right velocity sensor 8 is provided) to the left side thereof (side on which left velocity sensor 7 is provided).
Therefore, a velocity signal detected by the right velocity sensor 8 is an inlet-side velocity signal of the fluid to be measured flowing into the measurement tubes 2 and 3. A velocity signal detected by the left velocity sensor 7 is an outlet-side velocity signal of the fluid to be measured flowing from the measurement tubes 2 and 3.
Each of the left velocity sensor 7 and the right velocity sensor 8 which detect the vibration velocities may be an acceleration sensor.
The converter 5 of the Coriolis flowmeter has a block structure as illustrated in FIG. 11.
The converter 5 of the Coriolis flowmeter includes a drive control section 10, a phase measurement section 11, and a temperature measurement section 12.
That is, the converter 5 of the Coriolis flowmeter has an input and output port 15. A drive signal output terminal 16 included in the drive control section 10 is provided in the input and output port 15. The drive control section 10 outputs a predetermined mode signal, from the drive signal output terminal 16 to the vibrator 6 attached to the measurement tubes 2 and 3 to resonance-vibrate the measurement tubes 2 and 3.
The drive signal output terminal 16 is connected to a drive circuit 18 through an amplifier 17. The drive circuit generates a drive signal for resonance-vibrating the measurement tubes 2 and 3 and outputs the drive signal to the amplifier 17. The amplifier amplifies the input drive signal and outputs the drive signal to the drive signal output terminal 16. The drive signal output from the amplifier 17 is output from the drive signal output terminal 16 to the vibrator 6.
A left velocity signal input terminal 19 to which a detection signal of the vibration velocity generated on the left side of the measurement tubes 2 and 3 vibrated by the vibrator 6 is input is provided in the input and output port 15. The left velocity signal input terminal 19 is included in the phase measurement section 11.
A right velocity signal input terminal 20 to which a detection signal of the vibration velocity generated on the right side of the measurement tubes 2 and 3 vibrated by the vibrator 6 is input is provided in the input and output port 15. The right velocity signal input terminal 20 is included in the phase measurement section 11.
The phase measurement section 11 performs A/D conversion on the vibration signals of the pair of velocity sensors in the case where the predetermined mode signal is output from the drive signal output terminal 16 to the vibrator 6 attached to the measurement tubes 2 and 3 to vibrate the measurement tubes 2 and 3 by the vibrator 6, to thereby perform digital conversion processing, and then obtains a phase difference between the converted signals.
The left velocity signal input terminal 19 is connected to an input terminal of an amplifier 21. An output terminal of the amplifier 21 is connected to an A/D converter 22. The A/D converter 22 converts, into a digital value, an analog signal obtained by amplifying the vibration signal output from the left velocity signal input terminal 19 by the amplifier 21.
The A/D converter 22 is connected to a computing device 23.
Further, the right velocity signal input terminal 20 is connected to an input terminal of an amplifier 24. An output terminal of the amplifier 24 is connected to an A/D converter 25. The A/D converter 25 converts, into a digital value, an analog signal obtained by amplifying the vibration signal output from the right velocity signal input terminal 20 by the amplifier 24.
Further, the digital signal output from the A/D converter 25 is input to the computing device 23.
Further, a temperature signal input terminal 26 included in the temperature measurement section 11 to which a detection value from the temperature sensor 9 is input is provided in the input and output port 15. The temperature measurement section 12 performs tube temperature compensation based on the detection temperature obtained by the temperature sensor 9 which is provided in the measurement tubes 2 and 3 and detects an internal temperature of the measurement tubes 2 and 3.
A resistance type temperature sensor is generally used as the temperature sensor 9 to measure a resistance value, to thereby calculate a temperature.
The temperature signal input terminal 26 is connected to a temperature measurement circuit 27. The temperature measurement circuit 27 calculates the internal temperature of the measurement tubes 2 and 3 based on the resistance value output from the temperature sensor 9. The internal temperature of the measurement tubes 2 and 3 which is calculated by the temperature measurement circuit 27 is input to the computing device 23.
In the phase measurement method using the Coriolis flowmeter 1 as described above, vibration is applied in a primary mode, to the measurement tubes 2 and 3, from the vibrator 6 attached to the measurement tubes 2 and 3. When the fluid to be measured flows into the measurement tubes 2 and 3 while the vibration is applied, a phase mode is produced in the measurement tubes 2 and 3.
Therefore, the signal (inlet-side velocity signal) from the right velocity sensor 8 and the signal (outlet-side velocity signal) from the left velocity sensor 7 in the Coriolis flowmeter 1 are output as a form in which the two signals are superimposed on each other. A signal output as the form in which the two signals are superimposed on each other includes not only a flow rate signal but also a large number of unnecessary noise components. In addition, a frequency is changed depending on, for example, a change in density of the fluid to be measured.
Therefore, it is necessary to remove an unnecessary signal from the signals from the right velocity sensor 8 and the left velocity sensor 7. However, it is very difficult to remove the unnecessary signal from the signals from the right velocity sensor 8 and the left velocity sensor 7 to calculate the phase.
Further, the Coriolis flowmeter 1 is often required to have very-high-precision measurement and high-speed response. In order to satisfy such requirements, a computing device having very-complex computation and high-processing performance is necessary, and hence the Coriolis flowmeter 1 itself is very expensive.
Thus, the Coriolis flowmeter 1 requires an established phase difference measurement method using both an optimum filter always fit to a measurement frequency and a high-speed computing method.
In conventional phase difference measurement methods of calculating a flow rate, a filter processing method of removing noise is divided into a method using an analog filter and a method using a digital filter.
The method using the analog filter may be relatively inexpensive (see, for example, JP 02-66410 A and JP 10-503017 A). However, JP 02-66410 A and JP 10-503017 A have a limit to improve the performance of the filter, and hence, there is a problem that the filter is not sufficient for the Coriolis flowmeter.
In recent years, a large number of Coriolis flowmeters using digital signal processing have been developed, and the method using the digital filter has been developed as the filter processing method of removing noise in the conventional phase difference measurement methods of calculating the flow rate.
Examples of conventional types of the Coriolis flowmeters using digital signal processing include a method of measuring a phase using a Fourier transform (see, for example, JP 2799243 B) and a method of selecting an optimum table fit to an input frequency from filter tables including a notch filter and a band-pass filter to measure a phase (see, for example, JP 2930430 B and JP 3219122 B).
<<Phase Measurement Method using Fourier Transform>>
A converter of the Coriolis flowmeter based on the phase measurement method using the Fourier transform has a block structure as illustrated in FIG. 12.
In FIG. 12, the left velocity signal input terminal 19 provided in the input and output port 15 to which the detection signal of the vibration velocity (outlet-side velocity signal) which is generated on the left side of the measurement tubes 2 and 3 vibrated by the vibrator 6 and which is detected by the left velocity sensor 7 is input is connected to a low-pass filter 30. The low-pass filter 30 is a circuit for extracting, through a frequency filter, only a low-frequency left velocity signal (outlet-side velocity signal) from the left velocity signal (outlet-side velocity signal) output from the left velocity sensor 7 detecting the vibration velocity generated on the left side of the measurement tubes 2 and 3 in the case where the measurement tubes 2 and 3 are vibrated by the vibrator 6.
The low-pass filter 30 is connected to an A/D converter 31. The A/D converter 31 converts, into a digital signal, the left velocity signal which is the analog signal output from the low-pass filter 30. The left velocity signal obtained as the digital signal by conversion by the A/D converter 31 is input to a phase difference measurement unit 32.
The A/D converter 31 is connected to a timing generator 33. The timing generator 33 generates a timing of sampling M-times (M is natural number) the input frequency.
On the other hand, the right velocity signal input terminal 20 provided in the input and output port 15 to which the detection signal of the vibration velocity (inlet-side velocity signal) which is generated on the right side of the measurement tubes 2 and 3 vibrated by the vibrator 6 and which is detected by the right velocity sensor 8 is input is connected to a low-pass filter 34. The low-pass filter 34 is a circuit for extracting, through a frequency filter, only a low-frequency right velocity signal (inlet-side velocity signal) from the right velocity signal (inlet-side velocity signal) output from the right velocity sensor 8 detecting the vibration velocity generated on the right side of the measurement tubes 2 and 3 in the case where the measurement tubes 2 and 3 are vibrated by the vibrator 6.
The low-pass filter 34 is connected to an A/D converter 35. The A/D converter 35 converts, into a digital signal, the right velocity signal which is the analog signal output from the low-pass filter 34. The right velocity signal obtained as the digital signal by conversion by the A/D converter 35 is input to the phase difference measurement unit 32.
Further, the A/D converter 35 is connected to the timing generator 33. The timing generator 33 generates a timing of sampling M-times (M is natural number) the input frequency.
Further, the right velocity signal input terminal 20 provided in the input and output port 15 to which the detection signal of the vibration velocity (inlet-side velocity signal) which is generated on the right side of the measurement tubes 2 and 3 vibrated by the vibrator 6 and which is detected by the right velocity sensor 8 is input is connected to a frequency measurement unit 36. The frequency measurement unit 36 measures the frequency of the detection signal of the vibration velocity (inlet-side velocity signal) which is generated on the right side of the measurement tubes 2 and 3 vibrated by the vibrator 6 and which is detected by the right velocity sensor 8.
The frequency measurement unit 36 is connected to the timing generator 33. The frequency measured by the frequency measurement unit 36 is output to the timing generator 33. The timing of sampling M-times (M is natural number) the input frequency is generated by the timing generator 33 and output to the A/D converters 31 and 35.
The phase difference measurement unit 32, the timing generator 33, and the frequency measurement unit 36 are included in a phase measurement computing device 40.
In the phase measurement method using the Fourier transform as illustrated in FIG. 12, the input signal (inlet-side velocity signal) from the right velocity sensor 8 is first input to the frequency measurement unit 36 to measure a frequency. The frequency measured by the frequency measurement unit 36 is input to the timing generator 33. The timing of sampling M-times (M is natural number) the input frequency is generated by the timing generator 33 and input to the A/D converters 31 and 35.
Further, the detection signal of the vibration velocity (outlet-side velocity signal) which is generated on the left side of the measurement tubes 2 and 3 and obtained as the digital signal by conversion by the A/D converter 31 and the detection signal of the vibration velocity (inlet-side velocity signal) which is generated on the right side of the measurement tubes 2 and 3 and obtained as the digital signal by conversion by the A/D converter 35 are input to the phase difference measurement unit 32. The detection signals are Fourier-transformed by a discrete Fourier transform unit incorporated in the phase difference measurement unit 32 and a phase difference is computed based on a ratio between a real component and imaginary component of the converted signals.
<<Phase Measurement Method using Digital Filter>>
Converters of the Coriolis flowmeter based on the phase measurement method using the digital filter are described with reference to block structural diagrams illustrated in FIGS. 13 and 14.
Frequency selection means such as a notch filter or a band-pass filter is used as the digital filter. An S/N ratio of an input signal is improved using the frequency selection means such as the notch filter or the band-pass filter.
FIG. 13 illustrates a block structure of a converter of the Coriolis flowmeter using the notch filter as the digital filter.
The input and output port 15, the left velocity signal input terminal 19, the right velocity signal input terminal 20, the low-pass filters 30 and 34, and the A/D converters 31 and 35 as illustrated in FIG. 13 have the same structures as the input and output port 15, the left velocity signal input terminal 19, the right velocity signal input terminal 20, the low-pass filters 30 and 34, and the A/D converters 31 and 35 as illustrated in FIG. 12, respectively.
In FIG. 13, the A/D converter 31 is connected to a notch filter 51. The notch filter 51 selects a frequency based on the left velocity signal which is obtained as the digital signal by conversion by the A/D converter 31, so as to improve an S/N ratio of an input signal to be output.
The notch filter 51 is connected to a phase measurement unit 52. The phase measurement unit 52 measures a phase of the left velocity signal which is obtained as the digital signal by conversion and which is improved in S/N ratio by the notch filter 51.
Further, the notch filter 51 is connected to a frequency measurement unit 53. The frequency measurement unit 53 measures a frequency of the left velocity signal which is obtained as the digital signal by conversion and which is improved in S/N ratio by the notch filter 51.
The frequency measured by the frequency measurement unit 53 is input to the notch filter 51.
Further, the A/D converter 35 is connected to a notch filter 54. The notch filter 54 selects a frequency based on the left velocity signal which is obtained as the digital signal by conversion by the A/D converter 31, so as to improve an S/N ratio of an input signal to be output.
The notch filter 54 is connected to the phase measurement unit 52. The phase measurement unit 52 measures a phase of the right velocity signal which is obtained as the digital signal by conversion and which is improved in S/N ratio by the notch filter 54.
Further, the frequency measured by the frequency measurement unit 53 is input to the notch filter 54.
In FIG. 13, a clock 55 is used for synchronization, and input to the A/D converters 31 and 35 to synchronize the A/D converter 31 and the A/D converter 35 with each other.
The notch filters 51 and 54, the phase measurement unit 52, the frequency measurement unit 53, and the clock 55 are included in a phase measurement computing device 50.
FIG. 14 illustrates a block structure of a converter of the Coriolis flowmeter using the band-pass filter (BPF) as the digital filter.
The input and output port 15, the left velocity signal input terminal 19, the right velocity signal input terminal 20, the low-pass filters 30 and 34, and the A/D converters 31 and 35 as illustrated in FIG. 14 have the same structures as the input and output port 15, the left velocity signal input terminal 19, the right velocity signal input terminal 20, the low-pass filters 30 and 34, and the A/D converters 31 and 35 as illustrated in FIG. 13, respectively.
In FIG. 14, the A/D converter 31 is connected to a band-pass filter (BPF) 61. The band-pass filter 61 is a circuit for extracting, through a frequency filter, only a left velocity signal having a set frequency (outlet-side velocity signal) from the left velocity signal (outlet-side velocity signal) which is output from the left velocity sensor 7 detecting the vibration velocity generated on the left side of the measurement tubes 2 and 3 and which is obtained as the digital signal by conversion by the A/D converter 31 in the case where the measurement tubes 2 and 3 are vibrated by the vibrator 6.
The band-pass filter 61 is connected to a phase measurement unit 62. The phase measurement unit 62 measures a phase of the left velocity signal which is obtained as the digital signal by conversion and which is improved in S/N ratio by the band-pass filter 61.
Further, the band-pass filter 61 is connected to a frequency measurement unit 63. The frequency measurement unit 63 measures a frequency of the left velocity signal which is obtained as the digital signal by conversion by the A/D converter 31 and which is improved in S/N ratio by the band-pass filter 61.
The frequency measured by the frequency measurement unit 63 is input to the band-pass filter 61.
Further, the A/D converter 35 is connected to a band-pass filter 64. The band-pass filter 64 is a circuit for extracting, through a frequency filter, only a right velocity signal having a set frequency (inlet-side velocity signal) from the right velocity signal (inlet-side velocity signal) which is output from the right velocity sensor 8 detecting the vibration velocity generated on the right side of the measurement tubes 2 and 3 and which is obtained as the digital signal by conversion by the A/D converter 35 in the case where the measurement tubes 2 and 3 are vibrated by the vibrator 6.
The band-pass filter 64 is connected to the phase measurement unit 62. The phase measurement unit 62 measures a phase of the left velocity signal which is obtained as the digital signal by conversion and which is improved in S/N ratio by the band-pass filter 64.
The band-pass filter 64 is connected to the frequency measurement unit 63. The frequency measured by the frequency measurement unit 63 is input to the band-pass filter 64.
In FIG. 14, a clock 65 is used for synchronization, and a clock signal from the clock 65 is input to the A/D converters and 35 to synchronize the A/D converter 31 and the A/D converter 35 with each other.
The band-pass filters 61 and 64, the phase measurement unit 62, the frequency measurement unit 63, and the clock 65 are included in a phase measurement computing device 60.
In the phase measurement method using the Fourier transform as described in JP 2799243 B, when the input frequency of the input detection signal of the vibration velocity is constant, a phase measurement method having very-high-frequency selectivity may be performed because the Fourier transform is used for frequency selection.
However, in the method using the Fourier transform as described in JP 2799243 B, when the input frequency of the input detection signal of the vibration velocity is changed according to a density or a temperature, it is necessary to change the transform method or the sampling rate. Therefore, the computing cycle or the computing method is changed, and hence a measurement value is varied and thus unstabilized.
In addition, in the method using the Fourier transform as described in JP 2799243 B, when the input frequency of the input detection signal of the vibration velocity is changed according to the density or the temperature, it is necessary to accurately synchronize the sampling rate with the input frequency of the input vibration velocity signal, and hence a design is very complicated.
Therefore, there is a problem that, when the temperature of the fluid to be measured is rapidly changed or the density is rapidly changed by mixing air bubbles into the fluid, the measurement precision is extremely reduced.
Further, the method using the Fourier transform as described in JP 2799243 B has a problem that the number of computing processings becomes very large because of the execution of the Fourier transform.
In the methods of selecting the optimum table fit to the input frequency from the filter tables including the notch filter and the band-pass filter to measure the phase as described in JP 2930430 B and JP 3219122 B, when the sampling rate is held, the design may be simplified.
However, as in the method using the Fourier transform as described in JP 2799243 B, the phase measurement methods using the digital filter as described in JP 2930430 B and JP 3219122 B require a very large number of filter tables corresponding to changed input frequencies, and hence have a problem that memory consumption of a computing device is large.
In addition, the phase measurement methods using the digital filter as described in JP 2930430 B and JP 3219122 B have a problem that it is difficult to select the optimum filter in a case where the input frequency rapidly changes.
Further, the phase measurement methods using the digital filter as described in JP 2930430 B and JP 3219122 B have a problem that a very large number of computations is required to improve frequency selection performance.
The phase measurement methods using the digital filter as described in JP 2930430 B and JP 3219122 B have the following problems.
(1) The method cannot follow the change in input frequency at high precision. That is, it is very difficult to realize measurement in a case where the density of the fluid to be measured rapidly changes because of air bubble mixing.
(2) In order to improve the frequency selection performance, a very large number of computations are required. Therefore, it is difficult to realize high-speed response, and hence the method is unsuitable for batch processing for a short period of time.
(3) The memory consumption of the computing device is large, and hence the design is complicated. Therefore, a circuit structure and design are complicated and very disadvantageous in cost.
When all the factors are considered, in any of the conventional phase measurement methods including the digital filter processing, a noise of a frequency band other than the tube frequencies of the measurement tubes 2 and 3 is removed, and hence the switching of the filter table, the change of the computing method, and the change of the sampling rate are required to always follow the tube frequencies of the measurement tubes 2 and 3. Therefore, there is a problem that it is necessary to perform computation which is very complicated and lacks high-speed performance.
Thus, when the measurement tubes 2 and 3 are vibrated by the vibrator 6, it is very likely to generate a computing error in each variation of the input frequencies of the vibration velocity signals which are detected by the right velocity sensor 8 for detecting the vibration velocity generated on the right side of the measurement tubes 2 and 3 and the left velocity sensor 7 for detecting the vibration velocity generated on the left side of the measurement tubes 2 and 3, and hence there is a problem that measurement precision is very low.